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N~erous homes are located on the North St. Vrain Creek t lood P I a i n u ps tream 0 I Lyon S. ~~any 0 f the homeS wOO I d be f I cod ed by the Iflte""6dlatellegloIl<lIFlooc:;morelo'ouldbetlcodoobythe Standard Project Flood. From Reference Points Ib to 6b the areaS flooded by the two potential fl~s vary significantly, averaging 200 leet Io'lder for the larger flood Io'hich "ould inundate to an average Io'ldth of 550 feet. From Reference Point 6b to Lyons the flooded araas of the two floocs ~ore closely coincide, with excep- tions for short reachos at Rofer$ncc Points 9b and 12b. The aroa 'iUbject to flooding On Sooth St. VralnCroek is "lder t>ut th'-lro ilppearstobof"werhOlleslnthefloodplainthere. in Lyons, virtually all the town are/l befW'een the two br"neh streams is sub- jact to flooding. This vulnerability has been demonstrated to a degree during past floods. On the left bank of North St. Yroill Creek little flooding would occur upstreMlof Reference Point 20b, whereas downstream of this point the fiooded arail widens to from 400 to 650 feet from the stream bank. DownstrOaM from Lyons the two potential floods cover about the same areas. ThG area subject to flooding expands drastlcally downstream from Reference Polnt 12. At this location tho croek flows along the left edge of the flood plain. The right bank flood plain is IOlo'er than the flood plain adjacent to the ch"nnel. The flood waters separato and ~ portion flowsdowntherlghtsldeofthefloodplainleaYlng islendsln the center. Because the flows on each slde of the flood plain are somewhat separ~tod from each other two different flood orofiles are produced. An JQdltlonal set of reference points are prQYlded to show flood elevations on the right side of the flood plain. Cross soction 9-9 and 10-10 On plate 10 show the different levels of flooding. The relatIonship of the flolo's between tho left and rlght sides of the flood plain 15 in a delicate balance. ~ew r~ds, structures or other modifIcations to the flood plain could change the1100d pattern and flood laJldthetisnotnowilood9d. New construction should be flood proofod se'o'orol feet "bove the Flooded areas ond flood damages - The areas along St. Yrain Creek, both rural and InLyons,thatwOJld be flcodad by the Inter- mediate Regional and Standard Project Floods are Shown On plates 2 through 5. Water surface profiles for these floods were determined using bI.lck.."tercomputatlonsemplovingthaStand"rd Step Mvthod. The profIles, on plates 6 through 8, show the depths of flooding in the channel and the elev"tion of the water surfaces over the adjacent floodpL"in. Hedd losses at bridges were computed using deta in "Hydraulics of Bridge Waterways" put>lished bytheU. S. Departmen t of Ccmnerce. Flood t>oJndaries "ere plotted On the datai led topo- graphic mapping and then transferred to aerlel photographs in this report. Plates 9 and 10 portray cross sections across the channel and yalley and show ground ele'o'atlons and depths of overbank flood- ing. Reference points h/lve been prowlded to locate flood oleyatlons at Intervals along St. Vraln Creek. The reference points coincide with the location of the surveyed cross sections used in the study. The location of the reference points are shown Oil both the "Flooded Area"platesan<Jthe"Profilo"plates. Tables 5 thrOJgh 8 show reference point data including the eleYations of the streambed and the Intermodlate Reglon/ll and Standerd Project Floods. A bearIng is alSO proYided showing the orlentatlon Of the cross section through the reference point. Depths of flow tor the Intermediate Regionai and Standard Project Floods can be obtained from the proflles,crosssections,orroferenC<ltables. For any specific situation "hereaccur~yof flooded area delineatlon is neoded it Io'ill be necessary to take tho appropriate flood olevation from one of these sources dlld locate by $uryey th" correspond Ing grOJ~d eloyation on tho flood plain. 20 "